well, you could make a very simple one by just feeding a timer into series of counters that trigger as each one reachs it's max (ie, the 1's of the seconds would trigger the 10's of the seconds when it reaches 10 itself). to display the time, all you would need is four seven segments along with their drivers.
this clock would be pretty inaccurate tho unless you took special care to produce an accurate timer and also the only way to set the time would be to reset all the counters at 0h00 so that they would count on from then.
if you think that might be what your looking to build, give me a shout and i'll try to help you further.